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artículo con referato
"Temperature Evolution of Crystal Field Interactions Across the Jahn-Teller Transition in a La7/8Sr1/8MnO3 Single Crystal"
G. Alejandro, M.C.G. Passeggi, D. Vega, C.A. Ramos, M.T. Causa, M. Tovar and R. Senis
Phys. Rev. B 68(21) (2003) 214429/1-15
We present ESR, x-ray diffraction and dilatometry studies in a La7/8Sr1/8MnO3 single crystal. ESR experiments were performed in the paramagnetic regime (TC ~ 180 K), from 220 K to 570 K at a microwave frequency of 9.4 GHz. We measured the ESR linewidth as a function of temperature with the static magnetic field (H) parallel to the crystallographic directions [100] and [001], referred to the orthorhombic (Pbnm) axes. Detailed angular variations at constant temperature for H rotating in the (001), (010) and (-110) planes were also performed. The temperature dependence of the linewidth for fixed directions clearly reflects the changes in the symmetry of the crystal. At the Jahn Teller (JT) transition, a very noticeable decrease of the linewidth is observed. The angular dependence of the linewidth reveals an anisotropy that is temperature dependent. The amplitud of the variations decreases with T and almost dissapears above TJT ~ 280 K. However, there is a small remnant anisotropy present up to 400 K. We have carefully investigated the temperature dependence of this anisotropy across the Jahn-Teller transition. We showed that it is related with the evolution of the crystal field interactions. The symmetry of the ESR spectra is explained in terms of a spin Hamiltonian that includes isotropic Heisenberg superexchange, single ion-crystal field, and Dzyaloshinsky-Moriya antisymmetic exchange. The experimental angular variation of the ESR linewidth and its temperature dependence were fitted using the calculated expression for the linewidth as obtained from a perturbative Kubo-Tomita approach. From the ESR data we have obtained a ratio E/D = -0.4 for the spin Hamiltonian coefficients associated with the orthorhombic (E) and tetragonal (D) Jahn-Teller distortions of the MnO6 octahedra. The temperature dependence of the parameters of the model are presented, and the correlation with the dilatometry results are discussed.
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